Gestation is the period of
development during the carrying of an
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
, and later
fetus, inside
viviparous animals (the embryo develops within the parent). It is typical for
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, but also occurs for some non-mammals.
Mammals during pregnancy can have one or more gestations at the same time, for example in a
multiple birth.
The time interval of a gestation is called the ''
gestation period
In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once it ...
''. In
obstetrics
Obstetrics is the field of study concentrated on pregnancy, childbirth and the postpartum period. As a medical specialty, obstetrics is combined with gynecology under the discipline known as obstetrics and gynecology (OB/GYN), which is a surgi ...
, ''
gestational age'' refers to the time since the onset of the last
menses, which on average is
fertilization age
Human fertilization is the union of a human egg and sperm, occurring primarily in the ampulla of the fallopian tube. The result of this union leads to the production of a fertilized egg called a zygote, initiating embryonic development. Scientis ...
plus two weeks.
Mammals
In mammals, pregnancy begins when a
zygote (fertilized ovum)
implants
Implant can refer to:
Medicine
*Implant (medicine), or specifically:
**Brain implant
**Breast implant
**Buttock implant
**Cochlear implant
**Contraceptive implant
**Dental implant
**Fetal tissue implant
**Implantable cardioverter-defibrillator
** ...
in the female's
uterus and ends once the fetus leaves the uterus during labor or an
abortion (whether induced or spontaneous).
Humans
![Human Fertilization](https://upload.wikimedia.org/wikipedia/commons/5/5b/Human_Fertilization.png)
In humans, pregnancy can be defined clinically or biochemically. Clinically, pregnancy starts from first day of the mother's last period. Biochemically, pregnancy starts when a woman's
human chorionic gonadotropin (hCG) levels rise above 25 mIU/mL.
Human pregnancy can be divided into three trimesters, each approximately three months long: the first, second, and third trimester. The first trimester is from the last menstrual period through the 13th week, the second trimester is 14th–28/29th week, and the third trimester is 29/30th–42nd week.
Birth normally occurs at a
gestational age of about 40 weeks, though it is common for births to occur from 37 to 42 weeks.
Labor occurring prior to 37 weeks gestation is considered
preterm labor and can result from multiple factors, including previous preterm deliveries.
Prenatal care is important for the maintenance of a healthy pregnancy and surveillance of related complications. In high-income countries, prenatal care typically involves monthly visits during the first two trimesters, with an increasing number of visits closer to delivery. At these visits, healthcare providers will evaluate a variety of parental and fetal metrics, including fetal growth and heart rate,
birth defects, maternal blood pressure, among others.
After birth, health care providers will measure the baby's weight, vital signs, reflexes, head circumference, muscle tone, and posture to help determine the gestational age.
Various factors can influence the duration of gestation, including
diseases in pregnancy
Complications of pregnancy are health problems that are related to pregnancy. Complications that occur primarily during childbirth are termed obstetric labor complications, and problems that occur primarily after childbirth are termed puerperal di ...
and adequate prenatal care. The rates of morbidity and pre-existing diseases that predispose mothers to life-threatening, pregnancy-related complications in the United States are increasing.
The brunt of this burden is experienced by non-Hispanic Black women. Inaccessibility of prenatal care may partially explain this ongoing disparity.
Other factors that affect prenatal care utilization include socioeconomic status, insurance status, childcare, social support, housing, and immigration status.
Non-mammals
![Pregnant scorpion](https://upload.wikimedia.org/wikipedia/commons/9/9f/Pregnant_scorpion.jpg)
In viviparous
animals, the embryo develops inside the body of the mother, as opposed to outside in an
egg
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...
(
oviparity
Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and m ...
). The mother then gives live birth. The less developed form of viviparity is called
ovoviviparity
Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop insi ...
, in which the mother carries embryos inside eggs. Most
vipers exhibit ovoviviparity. The more developed form of viviparity is called
placental viviparity; mammals are the best example, but it has also evolved independently in other animals, such as in
scorpions, some
sharks, and in
velvet worms. Viviparous offspring live independently and require an external food supply from birth. Certain lizards also employ this method such as the genera ''Tiliqua'' and ''Corucia.'' The placenta is attached directly to the mother in these lizards which is called viviparous matrotrophy.
Ovoviviparous
animals develop within
egg
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...
s that remain within the mother's body up until they hatch or are about to hatch. It is similar to viviparity in that the embryo develops within the mother's body. Unlike the embryos of viviparous species, ovoviviparous
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s are nourished by the
egg yolk
Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...
rather than by the mother's body. However, the mother's body does provide
gas exchange. The young of ovoviviparous
amphibian
Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s are sometimes born as
larvae, and undergo
metamorphosis
Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...
outside the body of the mother.
The fish family
Syngnathidae has the unique characteristic whereby females lay their
eggs in a brood pouch on the male's chest, and the male incubates the eggs. Fertilization may take place in the pouch or before implantation in the water. Included in Syngnathidae are
seahorses, the
pipefish, and the
weedy and
leafy sea dragon
The leafy seadragon (''Phycodurus eques'') or Glauert's seadragon, is the only member of the genus ''Phycodurus'' and is a marine fish in the family Syngnathidae, which includes seadragons, pipefish, and seahorses.
It is found along the sou ...
s.
Syngnathidae is the only family in the animal kingdom to which the term "
male pregnancy" has been applied.
See also
*
Evolution of mammals (for the evolution of gestation-related features in humans and other mammals)
*
Male pregnancy
*
Nesting instinct
*
Pregnancy
*
Pregnancy in fish
Pregnancy has been traditionally defined as the period of time eggs are incubated in the body after egg-sperm union. Although the term often refers to placental mammals, it has also been used in the titles of many international, peer-reviewed, s ...
*
Prenatal development
Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...
*
Prenatal nutrition and birth weight
Prenatal nutrition addresses nutrient recommendations before and during pregnancy. Nutrition and weight management before and during pregnancy has a profound effect on the development of infants. This is a rather critical time for healthy develop ...
References
{{Reflist
External links
NSRL.ttu.edu
Reproduction in mammals